GammaShapeScale - changed the dispatch version of default_parametrization for Gamma

[skoghoern]

Attempt to solve #468 :
I changed the dispatch version of default_parametrization for Gamma to include a warning and then use the fallback version with lookup in ReactiveMP.
GraphPPL.default_parametrization(backend::ReactiveMPGraphPPLBackend, nodetype::GraphPPL.Atomic, factor::Type{Gamma}, rhs) = begin @warn "'Gamma' and 'GammaShapeScale' without keywords are constructed with parameters (Shape, Scale)." return GraphPPL.default_parametrization(backend, nodetype, ReactiveMP.inputinterfaces(factor), factor, rhs) end
This allows to define GammaShapeScale(1,1) as well as Gamma(1,1) both returning the warning as well as Gamma{Float64}(α=1.0, θ=1.0)

Also added tests for initialization macro (but also works for model macro). not sure whether we should add further tests there as well.
TO-DO: if this sounds cool, we need to update test nr. 9 of "convert_init_node_aliases" in my other PR:
# Test 9: Positional Gamma should warn and be treated as (shape, scale) input = quote μ(x) = Gamma(2.0, 1.0) end @test_logs (:warn, "'Gamma' and 'GammaShapeScale' without keywords are constructed with parameters (Shape, Scale).") apply_pipeline(input, convert_init_node_aliases)

[wouterwln]

This looks about right to me, thanks for the PR!